The present invention relates to a refrigerant gas heater system for warming portions of the cabinet structure of refrigerated display cases used in retail food and supermarket outlets. More particularly, it relates to a refrigerant gas heater system which utilizes heat energy added to the refrigerant gas by the compressor which is normally rejected and thus lost into the ambient environment. Thus, this invention utilizes energy produced during the normal refrigeration cycle to heat structural portions of the display cabinet in order to maintain those portions at temperatures above the dew point for ambient moisture conditions in order to prevent moisture condensation which could interfere with the operation and merchandise functioning of the cabinets.
The term "refrigerated," in accordance with the present invention, is intended to incorporate those display cabinets or cases maintained at a temperature of at least 32.degree. F., such as display cases utilized for display of milk or fresh foods, and those cases maintained below 32.degree. F., such as frozen foods cases. In addition, references are made herein to the use of transparent doors with such cabinets, since those are the types of doors most frequently used in such retail outlets. Other types of doors could also be employed within the scope of the present invention.
During the operation of the refrigerated display cabinet in retail food and supermarket outlets, structural portions of those cabinets act as condensation surfaces for ambient moisture, since the temperature of those surfaces is sometimes below the dew point at the prevailing ambient humidity conditions. Some of this condensation accumulates on the door jambs and mullions which are disposed about the door panels. This presents the possible dripping of water on the floor in front of the cabinet with resulting hazards for slipping. Another problem is that condensation on various surfaces can drip into the cabinets to cause operating and sanitation problems of the latter due to the promotion of bacterial growth. Cold moisture condensation is also objectionable to customers, particularly when their hands or clothing come in contact therewith as they select products from the display cabinets.
It is, therefore, considered desirable to provide heating for those structural portions of the cabinets which are subject to these condensation problems. One approach is to utilize electrical resistance heaters placed in contact with various cabinet structural parts. Another approach is to force an air stream over the surface for which condensate removal is desired. Both ambient air and heated air flow streams can be employed for this purpose, but the use of air flow streams is not practical to implement for all cabinet parts. Due to the increased cost of energy, efforts have been made to avoid the use of condensate removal systems which utilize additional energy inputs for the condensation removal, such as resistance heaters and supplemental fan systems.
Another approach for heating the structural portions of refrigerated cabinets most likely to encounter moisture condensate formation is the use of a hot liquid refrigerant line, particularly, a line taken from the refrigeration circuit immediately downstream of the condenser. Such liquid lines contain a low level of sensible heat which can be insufficient for preventing moisture condensate formation in situations where high ambient humidity and low operating of the refrigerated display cabinets are encountered. This problem is of increasing importance, since the art of refrigeration in display cabinets is moving toward the use of lower liquid refrigerant temperatures and lower compressor head pressures.
Another approach in the prior art is to use a hot refrigerant gas line as a heater. A problem encountered with this alternative is that the compressor gas is at an elevated and superheated temperature which may cause long gas lines containing such gas to buckle and even rupture. A typical compressor discharge terperature range is from 150.degree. F. to 240.degree. F., This temperature range will cause extensive buckling problems which then necessitates the use of relatively expensive heat-expansion loops and hanger systems and, particularly, when the heater tubing is placed in close contiguous heat exchange contact with various structural portions of the display cabinets, th expansion and contraction problems can be severe. Particularly, due to the fact that the heater system may not be employed continuously, metal in the heater lines will be alternatively heated by the hot refrigerant gas contained therein and cooled during time periods when flow of hot gas is interrupted. This effect is accentuated by the heater lines being cooled during the start-up of a refrigeration cycle by contact with cold air within the cabinet immediately following a defrost cycle. Effective heat transfer contact between the heater lines and the other structural parts is also difficult to maintain under these circumstances.
Another problem encountered with the use of hot gas heater lines is that temperatures in the above range can cause unpleasant experiences for customers including injury when fingers, hands, and forearms come in contact with the heated structural portions of the display cabinets. This is a problem particularly with respect to children who often touch these structural parts of display cases in food stores.
These general background statements are derived from several groups of prior art patents. Various air flow means to prevent condensation formation on refrigerator structures are shown in the following U.S. Pat. Nos. 2,673,455 to Brinkoeter; 2,672,735 to Fusselman; 2,706,387 to Swanson, 3,180,109 to Kimmel; and 4,009,586 to Skvarenia.
U.S. Pat. No. 3,371,503 to Perez, and assigned to the same assignee as the present application, discloses the use of a hot liquid line for heating several structural parts in refrigerated display cabinets. Other patents of this type are U.S. Pat. Nos. 2,657,546; 4,192,149 and 4,197,718. Liquid heater lines are also disclosed in co-pending U.S. patent application Ser. No. 36,661 filed May 7, 1979 by Fayez Ibrahim and Arthur Perez and assigned to the same assignee as the present application. Refrigerant liquid heater system such as shown in U.S. Pat. No. 3,839,879 for periodically removing frost and ice from the evaporator coils are distinquishable from the present invention due to the differences in problems perceived, concept, construction, mode of operation and result.
The use of refrigerant gas as a heat exchange medium, which encounters one or more of the above-referred to problems, is disclosed in U.S. Pat. Nos. 2,287,997 to Jarvis; 3,308,635 to Tenniswood; 3,835,660 to Franck; and 4,158,294 to Keeling. U.S. Pat. No. 2,960,884 to Quick discloses both an electric resistance heater and a hot gas heater for deicing a duct.
The prior art as represented by these patents does not appear to have recognized the problems inherent in the use of high temperature gas lines from the compressor with respect to the heating of refrigerated cabinet structural parts such as door mullions and jambs. The solutions found for these problems do not include the disclosed and claimed use of an adequate flow of low temperature refrigerent gas which has sufficient sensible heat and/or heat of condensation to provide the heating function and yet avoid the problems set forth stemming from overly high compressor gas discharge temperatures.
An advantage of the present invention is the energy conservation incurred by the use of refrigerant compressor gas to heat the structural parts of the display cabinet rather than using additional electrical input in order to heat these parts by resistance heaters. An average supermarket may employ about 96 feet of glass door case length for low temperature use and up to about 72 feet for medium temperature uses. The anticondensate input for a low temperature case is approximately 700 watts per 12 foot case during the refrigeration cycle. A medium temperature case utilizes about 400 watts per 12 foot case. Replacement of electrical resistance heaters for door mullions in such cases, for this average example, can result in a savings of 70,000 killowatts per year which translates to $3500 per year at a 5 cent per killowatt hour electric charge.